pkg/typify: empty messages are OtherError (#1531)

Messages with nothing in them are considered OtherError, they can not
serve any purpose for normal clients (i.e. dyn update or notifies might
have a use for them).

Also update a test in the cache plugin, so that we explicitaly test for
this case.

plugin/pkg/response/typify.go

@@ -55,6 +55,7 @@ func Typify(m *dns.Msg, t time.Time) (Type, *dns.OPT) {
 	if m == nil {
 		return OtherError, nil
 	}
+
 	opt := m.IsEdns0()
 	do := false
 	if opt != nil {
@@ -76,6 +77,11 @@ func Typify(m *dns.Msg, t time.Time) (Type, *dns.OPT) {
 		}
 	}
 
+	if m.Response && len(m.Answer) == 0 && len(m.Ns) == 0 {
+		// Response with nothing in it, maybe stuff in the additional section, this is not useful.
+		return OtherError, opt
+	}
+
 	// If our message contains any expired sigs and we care about that, we should return expired
 	if do {
 		if expired := typifyExpired(m, t); expired {

plugin/pkg/response/typify_test.go

@@ -26,6 +26,23 @@ func TestTypifyDelegation(t *testing.T) {
 	}
 }
 
+func TestTypifyEmptyMessage(t *testing.T) {
+	m := new(dns.Msg)
+
+	// Normal question, with response = false
+	m.SetQuestion("example.org.", dns.TypeAAAA)
+	mt, _ := Typify(m, time.Now().UTC())
+	if mt != NoError {
+		t.Errorf("message is wrongly typified, expected NoError, got %s", mt)
+	}
+	// In case of a Reponse = true, this weird.
+	m.Response = true
+	mt, _ = Typify(m, time.Now().UTC())
+	if mt != OtherError {
+		t.Errorf("message is wrongly typified, expected OtherError, got %s", mt)
+	}
+}
+
 func TestTypifyRRSIG(t *testing.T) {
 	now, _ := time.Parse(time.UnixDate, "Fri Apr 21 10:51:21 BST 2017")
 	utc := now.UTC()